1. Field of the Invention
The present invention relates to hetero-junction bipolar transistors (HBTs) manufactured by a metal-organic chemical vapor deposition (MOCVD) method and in particular to a method of manufacturing the same capable of improving properties of the HBTs.
2. Description of the Background Art
An HBT comprised of a collector of n-type GaAs, a base of p-type GaAs and an emitter of n-type AlGaAs has been increasingly studied and developed to be used as an ultra high speed electronic device.
An HBT used as a high current transistor for mobile communication is required to have a base layer having a hole carrier concentration of at least 2.times.10.sup.19 /cm.sup.3. Furthermore, an HBT used in milliwave communication in the future is required to have a still higher hole carrier concentration of at least 4.times.10.sup.19 /cm.sup.3. To obtain such high hole carrier concentrations, carbon has been noted as impurity atoms of the base layers. A main reason therefor is that carbon characteristically has a small diffusion constant in a crystal of a base layer and is also capable of heavy doping.
For example, Japanese Patent Laying-Open No. 9-50963 discloses that an MOCVD method using trimethyl (TM) Ga for a precursor for a group-III element, AsH.sub.3 for a precursor for a group-V element and CBr.sub.4 for a precursor for carbon is employed to grow a carbon-doped GaAs layer, which has achieved a hole carrier concentration of approximately 4.times.10.sup.19 /cm.sup.3.
FIG. 8 schematically shows a cross section of a semiconductor wafer for an HBT prepared by the inventors of the present application according to a conventional technique. It should be noted that in the figures, the same reference characters denote corresponding portions. The FIG. 8 semiconductor wafer includes a substantially insulated GaAs substrate 1 on which an n-type GaAs collector contact layer 2 of 0.5 .mu.m thickness with an electron carrier concentration of 5.times.10.sup.13 /cm.sup.3, an n-type GaAs collector layer 3 of 0.7 .mu.m thickness with an electron carrier concentration of 2.times.10.sup.16 /cm.sup.3, a carbon-doped, p-type GaAs base layer 4 of 0.1 .mu.m thickness with a hole carrier concentration of 2.times.10.sup.19 /cm.sup.3, an n-type Al.sub.0.3 Ga.sub.0.7 As emitter layer 5 of 0.1 .mu.m thickness with an electron carrier concentration of 5.times.10.sup.17 /cm.sup.3, and an n-type GaAs emitter contact layer 6 of 0.2 .mu.m thickness with an electron carrier concentration of 5.times.10.sup.18 /cm.sup.3 are successively, epitaxially grown using an MOCVD method.
In growing p-type GaAs base layer 4, at a temperature of 590.degree. C. the flow-rate ratio of TMAs for a precursor for a group-V element to TMGa for a precursor for a group-III element, i.e., V/III was set at 3.5 and the flow rate of TMGa was set at 1.5 sccm. The n-type Al.sub.0.3 Ga.sub.0.7 As emitter layer 5 and n-type GaAs emitter contact layer 6 were also grown at the temperature of 590.degree. C.
The FIG. 8 semiconductor wafer thus obtained according to a conventional technique was used to fabricate an HBT and measure it for current gain. As a result, it has been found that the HBT's current gain varies depending on the current flow, thermal stress and the like in the HBT. It had a current-gain variation ratio of 1.8, assuming that a current-gain variation ratio is defined as a current gain of an HBT after sufficient current flow that is divided by an initial current gain of the HBT.
According to a result of an experiment carried out by the inventors of the present application, operating a circuit as designed, including a high current transistor for mobile communication, requires a current-gain variation ratio of no more than 1.1, more preferably no more than 1.05. In other words, a circuit which includes an HBT used as a high current transistor for mobile communication will not operate as designed if the HBT is fabricated using the FIG. 8 semiconductor wafer obtained according to a conventional technique.